Multilayer polymeric reflector

ABSTRACT

A multilayer polymeric reflector is provided which comprises: a) a plurality of first optical layers, each first optical layer comprising a polyester having terephthalate comonomer units and ethylene glycol comonomer units, the polyester having a glass transition temperature, where each first optical layer is oriented, and b) a plurality of second optical layers disposed in a repeating sequence with the plurality of first optical layers, each second optical layer comprising a blend of polymethyl methacrylate (PMMA) and polyvinylidene fluoride (PVDF), where the blend has a glass transition temperature less than the glass transition temperature of the polyester comprising the first optical layers, and where the amount of PVDF in the PMMA/PVDF blend is greater than and not equal to about 40% and not more than about 65%. Articles comprising the multilayer polymeric reflector are also provided.

FIELD OF THE DISCLOSURE

This disclosure relates to multilayer polymeric reflectors which areparticularly useful in connection with high intensity light sources dueto their durability and resistance to yellowing.

BACKGROUND OF THE DISCLOSURE

U.S. Pat. No. 7,141,297 discloses a multilayer polymeric mirrorcomprising oriented layers of PET alternating with layers of a PMMA/PVDFblend that has a Tg lower than the PET. That reference teaches:

-   -   “The amount of PVDF used in the blends is typically not more        than about 40% by weight (i.e. a 60/40 PMMA/PVDF blend). With        higher levels of PVDF, the miscibility of the PMMA and PVDF        tends to deteriorate, thereby causing losses in clarity. In        general, it is desirable to use PVDF in the blends in an amount        as high as possible in order to increase the benefit in        reductions in refractive index and glass transition temperature.        However, smaller amounts can be used when it is desired to fine        tune the composition to provide particular optical or physical        properties for certain applications. For example, a 75/25 blend        provides highly desirable physical and optical properties for        use with high refractive index materials such as PEN, PET and        mixtures or copolymers thereof.” (U.S. Pat. No. 7,141,297 at        col. 14, lines 42-55)    -   “As described above, the blending of polyvinylidene fluoride        (PVDF) with PMMA reduces the glass transition temperature of the        blended polymers. Preferably, the blend includes about 20 to 40        wt. % PVDF and 60 to 80 wt. % PMMA. Below about 20 wt. % PVDF,        the glass transition temperature is above that of PET, although        these blends are still acceptable for some applications. Above        about 40 wt. %, PVDF crystallizes. The addition of PVDF to the        second optical layers can also enhance other properties, such        as, for example, solvent resistance.” (U.S. Pat. No. 7,141,297        at col. 14, line 65-col. 15, line 7.)

SUMMARY OF THE DISCLOSURE

Briefly, the present disclosure provides a multilayer polymericreflector comprising: a) a plurality of first optical layers, each firstoptical layer comprising a polyester having terephthalate comonomerunits and ethylene glycol comonomer units, the polyester having a glasstransition temperature, where each first optical layer is oriented, andb) a plurality of second optical layers disposed in a repeating sequencewith the plurality of first optical layers, each second optical layercomprising a blend of polymethyl methacrylate (PMMA) and polyvinylidenefluoride (PVDF), where the blend has a glass transition temperature lessthan the glass transition temperature of the polyester comprising thefirst optical layers, and where the amount of PVDF in the PMMA/PVDFblend is greater than and not equal to about 40% and not more than about65%. The multilayer polymeric reflector has a reflectivity of greaterthan 97.8% in a visible wavelength region and a transmission haze valueof less than 50% in a visible wavelength region. In some embodiments,the amount of PVDF in the PMMA/PVDF blend is greater than 45%, and insome embodiments the amount of PVDF in the PMMA/PVDF blend is greaterthan or equal to about 50%. In some embodiments, the amount of PVDF inthe PMMA/PVDF blend is about 50%. In some embodiments, the multilayerpolymeric reflector has a reflectivity of greater than 98.0%, in someembodiments, greater than 98.2%. In some embodiments, the total numberof first and second layers is no more than 700, in some embodiments, nomore than 650. In some embodiments, the multilayer polymeric reflectorresists shrinkage in use, to the extent that it demonstrates shrinkageof less than 1.5% in the total of width plus length following anexposure of 15 minutes to a temperature of 120 degrees centigrade; insome embodiments, the multilayer polymeric reflector demonstratesshrinkage of less than 1.0%, in some less than 0.5%, and in some lessthan 0.2%. In some embodiments, the multilayer polymeric reflector has atransmission haze value of less than 46% in a visible wavelength region,in some less than 42%, in some less than 30%, in some less than 20%, andin some less than 10%. In some embodiments, the first and second opticallayers are coextruded. In some embodiments, the first optical layers arebiaxially oriented. In some embodiments, the multilayer polymericreflector is annealed at an annealing temperature of between 70 and 95degrees centigrade for at least 30 seconds. In some embodiments, themultilayer polymeric reflector is annealed at an annealing temperatureof between 80 and 95 degrees centigrade for at least 30 seconds. In someembodiments, the multilayer polymeric reflector is annealed at anannealing temperature of between 80 and 95 degrees centigrade for atleast 35 seconds. In some embodiments, the multilayer polymericreflector is annealed at an annealing temperature of between 70 and 95degrees centigrade for at least two minutes. In some embodiments, themultilayer polymeric reflector is annealed at an annealing temperatureof between 70 and 95 degrees centigrade for at least one hour. In someembodiments, the multilayer polymeric reflector additionally comprisesan optically clear UV-rejecting acrylic coating layer. In someembodiments, the multilayer polymeric reflector additionally comprisesan adhesive layer. In some embodiments, the multilayer polymericreflector is specular or semi-specular at visible wavelengths. In someembodiments, the multilayer polymeric reflector is at least 50% specularat visible wavelengths. In some embodiments, the multilayer polymericreflector is installed for outdoor use; in some embodiments it isdirectly exposed to ambient outdoor light, and in some embodiments it isdirectly exposed to ambient outdoor light and ambient outdoor air.

In another aspect, the present disclosure provides an articlecomprising: a) a light source; and b) the multilayer polymeric reflectoraccording to the present disclosure. In some embodiments, the lightsource is an LED. In some embodiments, the article is a luminaire. Insome embodiments, the article is a light bulb.

In another aspect, the present disclosure provides an article comprisingthe multilayer polymeric reflector according to the present disclosuresituated so as to receive and reflect sunlight; in some embodimentdirect sunlight, and in some embodiments concentrated sunlight.

In some embodiments, an article according to the present disclosure isdesigned for outdoor use. In some embodiments, the article is installedfor outdoor use. In some embodiments, the multilayer polymeric reflectorof the article is directly exposed to ambient outdoor light. In someembodiments, the multilayer polymeric reflector of the article isdirectly exposed to ambient outdoor light and ambient outdoor air.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph demonstrating UV light resistance of multilayerpolymeric reflector material according to the present disclosure andcomparative material, as described in the Examples.

FIG. 2 is a graph demonstrating shrinkage of multilayer polymericreflector material according to the present disclosure and comparativematerial, as described in the Examples.

FIG. 3 is a graph demonstrating haze level of multilayer polymericreflector material according to the present disclosure, as described inthe Examples.

FIG. 4a and FIG. 4b are micrographs of multilayer polymeric reflectormaterial according to the present disclosure demonstrating reduction ofhaze by addition of a clear UV coat, as described in the Examples.

DETAILED DESCRIPTION

The present disclosure provides a multilayer polymeric reflectorcomprising: a) a plurality of first optical layers, each first opticallayer comprising a polyester having terephthalate comonomer units andethylene glycol comonomer units, said polyester having a glasstransition temperature, wherein each first optical layer is oriented,and b) a plurality of second optical layers disposed in a repeatingsequence with the plurality of first optical layers, each second opticallayer comprising a blend of polymethyl methacrylate (PMMA) andpolyvinylidene fluoride (PVDF), wherein said blend has a glasstransition temperature less than the glass transition temperature of thepolyester comprising the first optical layers, and wherein the amount ofPVDF in the PMMA/PVDF blend is greater than and not equal to about 40%and not more than about 65%; wherein the multilayer polymeric reflectorhas a reflectivity of greater than 97.8% in a visible wavelength regionand a transmission haze value of less than 50% in a visible wavelengthregion. The present disclosure also provides articles that include suchreflectors, in particular articles wherein the reflector is positionedin close proximity to a light source, articles wherein the reflector isused to receive and reflect sunlight, or articles used outdoors withexposure to sunlight.

The present disclosure concerns a multilayer polymeric reflectorcomprising oriented layers of PET alternating with layers of a PMMA/PVDFblend, where the PMMA/PVDF ratio is less than 60/40 and most typically50/50. The present reflector achieves a reflectivity of 97.8% orgreater. Without wishing to be bound by theory, the authors believe thatthe present reflector achieves this result without the loss of claritypredicted in the prior art due to the use of an annealing step duringmanufacture. In many embodiments, the reflector of the presentdisclosure also demonstrates superior resistance to yellowing. In someembodiments, the mirror may be subjected to additional heat treatmentafter annealing to reduce the potential for shrinkage in use, butwithout introducing unacceptable levels of haze. Finally, in someembodiments the addition of an optically clear UV-rejecting acryliccoating further increases resistance to yellowing and acts to prevent aseparate mechanism of hazing in the PET polymer.

The present invention is generally directed to multilayer polymericreflectors that are light-reflecting multilayer optical films (such asmultilayer optical films) and their manufacture, as well as the use ofthe multilayer optical films as mirrors and in light directing and lightproviding articles. These multilayer optical films include multilayeroptical films, methods of making and using these multilayer opticalfilms, and articles incorporating the multilayer optical films. Themultilayer optical films reflect light over a wavelength range (e.g.,all or a portion of the visible, IR, or UV spectrum, but most typicallyall or a portion of the visible wavelength range). The multilayeroptical films are typically coextruded and oriented multilayerstructures that differ from previous optical bodies, at least in part,due to the selection of materials which can provide advantages inprocessing, optical properties, mechanical properties, durability,weatherability, and economic and other advantages. While the presentinvention is not so limited, an appreciation of various aspects of theinvention will be gained through a discussion of the examples providedbelow.

The term “birefringent” means that the indices of refraction inorthogonal x, y, and z directions are not all the same. For the polymerlayers described herein, the axes are selected so that x and y axes arein the plane of the layer and the z axis is normal to the plane of thelayer and typically corresponds to the thickness or height of the layer.For an oriented polymer, the x-axis is generally chosen to be thein-plane direction with the largest index of refraction, which typicallycorresponds to one of the directions in which the optical body isoriented (e.g., stretched). The term “in-plane birefringence” is theabsolute value of the difference between the in-plane indices (n_(x) andn_(y)) of refraction.

The term “polymer” will be understood, unless otherwise indicated, toinclude polymers and copolymers (i.e., polymers formed from two or moremonomers including terpolymers, etc.), as well as polymers or copolymerswhich can be formed in a miscible blend by, for example, coextrusion orreaction, including transesterification. Block, random, graft, andalternating copolymers are included, unless indicated otherwise.

All birefringence and index of refraction values are reported for 632.8nm light unless otherwise indicated.

Multilayer Optical Films

Certain multilayer optical films are described in U.S. Pat. No.7,141,297 and references cited therein, all of which are incorporatedherein by reference.

Multilayer optical films that can be used in the present disclosureinclude one or more first optical layers, one or more second opticallayers, and may optionally include one or more non-optical layers. Thenon-optical layers can be disposed on a surface of the multilayeroptical film as a skin layer or disposed between optical layers. Thefirst and second optical layers and, optionally, the non-optical layers,if any, are coextruded and oriented by, for example, stretching.Orientation typically significantly enhances the optical power (e.g.,reflectivity) of the multilayer optical films due to birefringence ofthe first or second optical layers or both.

The optical layers are typically interleaved to form a stack of layers,optionally, with one or more of the non-optical layers included withinor as a skin layer of the stack. Typically the optical layers arearranged as alternating pairs to form a series of interfaces betweenlayers with different optical properties. The optical layers aretypically no more than 1 □m thick and can have a thickness of 400 nm orless. The optical layers can have the same thicknesses. Alternatively,the multilayer optical film can include layers with differentthicknesses to increase the reflective wavelength range.

Multilayer optical films can have a large number of optical layers.Examples of suitable multilayer optical films include those having about2 to 5000 optical layers. Generally, multilayer optical films have about25 to 700 optical layers and typically about 50 to 700 optical layers orabout 75 to 700 optical layers. In some embodiments, the multilayeroptical films contain no more than 700 first and second optical layers.In some embodiments, the multilayer optical films contain no more than650 first and second optical layers. It will be appreciated that themultilayer optical film can be made from multiple stacks that aresubsequently combined to form the optical body.

Additional sets of optical layers, similar to the first and secondoptical layers, can also be used in the multilayer optical film. Thedesign principles disclosed herein for the sets of first and secondoptical layers can be applied to any additional sets of optical layers.In addition, different repeating patterns of optical layers can be used(e.g., “ABCBA . . . ”, where A, B, and C are optical layers withdifferent compositions). Some such patterns as set forth in U.S. Pat.No. 5,360,569, which is incorporated herein by reference.

The transmission and reflection characteristics of the multilayeroptical films are based on coherent interference of light caused by therefractive index difference between the first and second optical layersand the thicknesses of the first and second optical layers. When thein-plane indices of refraction differ between the first and secondoptical layers, the interface between adjacent first and second opticallayers forms a reflecting surface. The reflective power of the interfacedepends on the square of the difference between the in-plane indices ofrefraction of the first and second optical layers (e.g., (n₁₀-n₂₀)²,where n₁₀ is an in-plane refractive index of the first optical layersand n₂₀ is an in-plane refractive index of the second optical layers).

In mirror applications, the multilayer optical film typically includesfirst and second optical layers where both in-plane refractive indicesdiffer substantially (e.g., differ by at least 0.04 and, often, by atleast 0.1) between the layers (i.e., n_(1x)≠n_(2x) and n_(1y)≠n_(2y),where n_(1x) and n_(1y) are the in-plane refractive indices of the firstoptical layers and n_(2x) and n_(2y) are the in-plane refractive indicesof the second optical layers). In polarizer applications, the multilayeroptical film typically includes first and second layers where one of thein-plane refractive indices differs substantially between the layers andthe other in-plane refractive index is substantially similar (e.g.,n_(1x)≠n_(2x) and n_(1y)≈n_(2y)). Preferably, the substantially similarin-plane refractive indices differ by no more than about 0.04. Forpolarizer applications, the in-plane birefringence of the first opticallayers is typically at least about 0.05, preferably at least about 0.15,and more preferably at least about 0.2.

The first optical layers are made using birefringent polymers(preferably, polymers with positive birefringence) that are uniaxially-or, preferably, biaxially-oriented to increase the in-plane refractiveindex (or indices) of the first optical layers, thereby increasing thedifference between the refractive indices of the first and secondlayers. In some embodiments, the second optical layers are polymerlayers that are birefringent (preferably, negatively birefringent) anduniaxially- or biaxially-oriented. In other embodiments, the secondoptical layers are polymer layers having an isotropic index ofrefraction (e.g., substantially the same index of refraction in alldirections) that is typically different from one or both of the in-planeindices of refraction of the first optical layers.

The first optical layers can be made birefringent by, for example,stretching the first optical layers in a desired direction ordirections. For example, the first optical layers can bebiaxially-oriented by stretching in two different directions. Thestretching of optical layers in the two directions can result in a netsymmetrical or asymmetrical stretch in the two chosen orthogonal axes.Symmetrical stretching in two directions can yield in-plane refractiveindices that are substantially similar (e.g., differ by no more than0.4). As an alternative to stretching in two directions, the firstoptical layers can be uniaxially-oriented by, for example, stretchingthe layers in a single direction. A second orthogonal direction may beallowed to neck (e.g., decrease in length, width, or thickness) intosome value less than its original length. The direction of stretchingtypically corresponds to either in-plane axis (e.g. the x or y axis),however, other directions can be chosen.

Typically, the highest reflectivity for a particular interface betweenfirst and second optical layers occurs at a wavelength corresponding totwice the combined optical thickness of the pair of optical layers. Theoptical thickness describes the difference in path length between lightrays reflected from the lower and upper surfaces of the pair of opticallayers. For light incident at 90 degrees to the plane of the opticalfilm (normally incident light), the optical thickness of the two layersis n₁d₁+n₂d₂ where n₁, n₂ are the in-plane indices of refraction of thetwo layers and d₁, d₂ are the thicknesses of the corresponding layers.The equation λ/2=n₁d₁+n₂d₂ can be used to tune the optical layers fornormally incident light. At other angles, the optical distance dependson the distance traveled through the layers (which is larger than thethickness of the layers) and the indices of refraction for at least twoof the three optical axes of the layer. The optical layers can each be aquarter wavelength thick or the optical layers can have differentoptical thicknesses, as long as the sum of the optical thicknesses ishalf of a wavelength (or a multiple thereof). A multilayer optical filmhaving more than two optical layers can include optical layers withdifferent optical thicknesses to provide reflectivity over a range ofwavelengths. For example, a multilayer optical film can include pairs orsets of layers that are individually tuned to achieve optimal reflectionof normally incident light having particular wavelengths or may includea gradient of layer pair thicknesses to reflect light over a largerbandwidth.

These multilayer optical films can be designed to reflect one or bothpolarizations of light over at least one bandwidth. The layerthicknesses and indices of refraction of the optical stacks within theoptical bodies can be controlled to reflect at least one polarization ofspecific wavelengths of light (at a particular angle of incidence) whilebeing transparent over other wavelengths. Through careful manipulationof these layer thicknesses and indices of refraction along the variousoptical body axes, the multilayer optical film of the present inventionmay be made to behave as mirrors or polarizers over one or more regionsof the spectrum.

For example, the optical bodies can be designed to reflect light oversubstantially all of the visible light region (about 400 to 750 nm) toform a visible mirror. The visible mirror may be a cold mirror,reflecting only the visible wavelengths of light and transmitting theIR, or it may be a broadband mirror that reflects both the visible andIR wavelengths. Visible mirrors are described, for example, in U.S. Pat.No. 5,882,774 and WO 97/01774, and a cold mirror is described, forexample, in U.S. Pat. Nos. 5,339,198 and 5,552,927, all of which areincorporated herein by reference. For cold mirrors, the typical opticallayer thickness is in the range of 100 to 200 nm. For visible/IRmirrors, the typical optical layer thickness is in the range of 100 to600 nm (for a ¼ wavelength design).

Another embodiment is an optical body that reflects at least a portionof infrared (IR) light. To reflect light in the region from about 750 to1200 nm, the layers have optical thicknesses ranging from about 185-300nm (¼ the wavelength of the light desired to be reflected). For example,the optical bodies of the present invention can be tuned to reflect bothpolarizations of light in at least a portion of the IR region of thespectrum while being transparent over other portions of the spectrum.This type of optical body can be used as an IR film to, for example,reflect solar energy from, for example, windows of buildings andautomobiles. Preferably, IR films for these uses transmit a largeportion of the visible light and, more preferably, have substantiallyuniform transmission spectra over the visible range to avoid theappearance of color. Further description of IR films and examples offilm configurations are presented in WO 97/01778, WO 99/36808, and U.S.Pat. No. 5,360,659, all of which are incorporated herein by reference.

Yet another embodiment is a multilayer optical film that reflects lightover only a portion of the visible range. These optical bodies can beused as color shifting films, because as viewing angle changes, thewavelength region of reflection also changes. Further description ofcolor changing films, principles of operation, and examples of filmconfigurations are presented in WO 99/36257 and WO 99/36258, both ofwhich are incorporated herein by reference. These optical bodies can betailored to exhibit a sharp bandedge at one or both sides of at leastone reflective bandwidth, thereby giving a high degree of colorsaturation at acute angles, if desired, as described in WO 99/36809,incorporated herein by reference.

First Optical Layers

The first optical layers are typically orientable films of polyethyleneterephthalate (PET) or copolymers or blends thereof. Examples ofsuitable copolymers are described in, for example, WO 99/36262, and inco-pending U.S. patent application Ser. No. 09/399,531, both of whichare incorporated herein by reference.

Preferred properties of the material used for the first optical layersinclude: 1) birefringence (preferably, positive birefringence), 2)thermal stability, 3) processing temperatures compatible with thematerials of the second optical layers, 4) UV stable or protectable, 5)high clarity (e.g., high transmission and low absorption), 6) a glasstransition temperature that is compatible with the second optical layersto provide strain-induced birefringence, 7) a range of viscosities topermit viscosity matching with the materials of the second opticallayers, 8) good interlayer adhesion with the second optical layers, 9)low dispersion, 10) good z-index matching with the second opticallayers, and 11) drawability (e.g., the ability to be stretched). Otherfactors include cost and commercial availability.

PET and copolymers and blends of PET as well as the other polymerslisted above, can be made birefringent by, for example, stretching thefirst optical layers in a desired direction or directions. Orientationis typically accomplished at a temperature above the glass transitiontemperature of the polymer, although some copolymers with lowcrystallinity can be oriented at or below the glass transitiontemperature as described in, for example, co-pending U.S. patentapplication Ser. No. 09/399,531, incorporated herein by reference.

Uniaxial orientation of polyethylene terephthalate (PET) can raise therefractive index of PET in the orientation direction from 1.57 to 1.69.Biaxial orientation of PET can raise the refractive index of PET in theorientation directions from 1.57 to 1.65, while the z index ofrefraction decreases to 1.50, giving a birefringence of 0.13 to 0.15between the in-plane and z-axis refractive indices.

The amount of birefringence and the amount of change in refractive indexobtained for these polymers depends on a variety of factors including,for example, the draw ratio, the orientation temperature, and whetherthe polymer is uniaxially or biaxially oriented. Typically, the largerthe draw ratio, the larger the change in refractive index. However, theachievable draw ratio can be limited by the orientation temperature.

Typically, for relatively crystalline materials, the orientationtemperature is above the glass transition temperature. Generally, thecloser that the orientation temperature is to the glass transitiontemperature, the lower the achievable draw ratio because the polymerexhibits excessive strain hardening when drawn and can crack or formmicrovoids. However, in general, the closer that the orientationtemperature is to the glass transition temperature, the large the changein refractive index for a given draw ratio. Thus, drawing the polymer ata temperature that is substantially above (e.g., 20° C. or 30° C.) theglass transition temperature of the polymer will typically result insignificantly less change in the refractive index for a given drawratio. Thus, a balance is required between draw ratio and orientationtemperature to achieve a desired refractive index change.

Material selection can influence the optical and physical properties ofthe multilayer optical film. Polyesters like PET include carboxylate andglycol subunits and can be generated by, for example, (a) reaction ofcarboxylate monomers with glycol monomers or (b) transesterification.Each carboxylate monomer has two or more carboxylic acid or esterfunctional groups and each glycol monomer has two or more hydroxyfunctional groups. Polyesters can be formed using a single type ofcarboxylate monomer or two or more different types of carboxylatemonomers. The same applies to the glycol monomers.

The properties of a polymer layer or film vary with the particularchoice of monomers. PET includes carboxylate subunits formed fromterephthalic acid or esters thereof.

PET includes glycol subunits formed using ethylene glycol. Suitableglycol comonomers for forming glycol subunits of PET include propyleneglycol; 1,4-butanediol and isomers thereof, 1,6-hexanediol; neopentylglycol; polyethylene glycol; diethylene glycol; tricyclodecanediol;1,4-cyclohexanedimethanol and isomers thereof, norbornanediol;bicyclo-octanediol; trimethylol propane; pentaerythritol;1,4-benzenedimethanol and isomers thereof; bisphenol A; 1,8-dihydroxybiphenyl and isomers thereof; and 1,3-bis(2-hydroxyethoxy)benzene.

On the other hand, PET absorbs light at 320 nm with a tail extending to370 nm. Thus, a UV protecting coating or additive would not need toextend into the visible range. This ability is particularly importantwhen preparing multilayer optical films that are designed to reflect IRlight and transmit visible light (e.g., solar reflective films forbuilding and automobile windows) or optical bodies designed to reflectonly a particular bandwidth in the visible range and transmitting allother light.

Suitable PET-containing multilayer optical films can be formed in avariety of configurations. Particularly useful PET-based materialsinclude PET or PET copolymers or blends that have a glass transitiontemperature of no more than about 90° C., or of no more than about 80°C. or 70° C. Typically, the most useful of these PET-based materialswill be free, or substantially free, of napthalene dicarboxylate (NDC)monomers. In such constructions, the material for the second opticallayers generally will also include a material having a glass transitiontemperature of no more than about 90° C. Among the materials forsuitable second optical layers are polyacrylates and aliphaticpolyolefins, including blends of these polymers with other materials andcopolymers. Alternatively, the first optical layers can be formed usinga copolymer or blend of PET that is also substantially free of NDCmonomer and that has a glass transition temperature of at least about100° C. or at least 120° C. In such constructions, the material for thesecond optical layers generally will also include a material having aglass transition temperature of at least about 100° C.

As an alternative, the glass transition temperature of PET can be raisedby combining PET with a second polymer that has a higher glasstransition temperature. The combination of PET and the second polymercan include miscible blending to form a polymer blend or reactiveblending (by, for example, transesterification) to form a copolymer. Forexample, PET can be blended with a second polymer that has a glasstransition temperature of 130° C. or higher or a second polymer with aglass transition temperature of 160° C. or higher, or even a secondpolymer with a glass transition temperature of 200° C. or higher.Examples of suitable second polymers include, for example, PEN(T_(g)=130° C.), polycarbonate (T_(g)=157° C.), polyarylate (T_(g)=193°C.), or polyetherimide (T_(g)=218° C.).

Alternatively, the monomer materials of PET, e.g., terephthalic acid andethylene glycol, can be copolymerized with the monomer materials thatcorrespond to a second polymer having a higher glass transitiontemperature, such as PEN, polycarbonate, and polyarylate, to formcopolymers. For example, PET can be copolymerized with monomer materialsthat correspond with a second polymer that has a glass transitiontemperature of 130° C. or higher or a second polymer with a glasstransition temperature of 160° C. or higher, or even a second polymerwith a glass transition temperature of 200° C. or higher.

Other copolymers of PET can also be used, including those incorporating(i) carboxylate monomer materials, such as, for example, isophthalicacid; phthalic acid; azelaic acid; adipic acid; sebacic acid; norbornenedicarboxylic acid; bi-cyclooctane dicarboxylic acid; 1,6-cyclohexanedicarboxylic acid and isomers thereof; t-butyl isophthalic acid;tri-mellitic acid; sodium sulfonated isophthalic acid; 2,2′-biphenyldicarboxylic acid and isomers thereof; and lower alkyl esters of theseacids, such as methyl or ethyl esters; and (ii) glycol monomermaterials, such as, for example, propylene glycol; 1,4-butanediol andisomers thereof; 1,6-hexanediol; neopentyl glycol; polyethylene glycol;diethylene glycol; tricyclodecanediol; 1,4-cyclohexanedimethanol andisomers thereof; norbornanediol; bicyclo-octanediol; trimethylolpropane; pentaerythritol; 1,4-benzenedimethanol and isomers thereof;bisphenol A; 1,8-dihydroxy biphenyl and isomers thereof; and1,3-bis(2-hydroxyethoxy)benzene.

Second Optical Layers

The second optical layers can be prepared with a variety of optical andphysical properties depending, at least in part, on the desiredoperation of the film. Preferred properties of the second optical layersinclude, for example, 1) isotropic or negative birefringence, 2) thermalstability, 3) processing temperatures compatible with the materials ofthe first optical layers, 4) UV stable or protectable, 5) high clarity(e.g., high transmission and low absorption), 6) a glass transitiontemperature that is compatible with the first optical layers to providestrain-induced birefringence, 7) a range of viscosities to permitviscosity matching with the materials of the first optical layers, 8)good interlayer adhesion with the first optical layers, 9) lowdispersion, 10) good z-index matching with the first optical layers, and11) drawability (e.g., the ability to be stretched). Other factorsinclude cost and commercial availability.

In some embodiments, the second optical layers are made of a polymermaterial that does not appreciably optically orient when stretched underconditions that are used to orient the first optical layers. Such layersare particularly useful in the formation of reflective optical bodies,because they allow the formation of a stack of layers by, for example,coextrusion, which can then be stretched to orient the first opticallayers while the second optical layers remain relatively isotropic(e.g., an in-plane birefringence of 0.05 or less). In other embodiments,the second optical layers are orientable and are, preferably, negativelybirefringent (when the first optical layers are positively birefringent)so that the in-plane refractive indices decrease with orientation.

There are a variety of considerations in the selection of the materialsfor the first and second optical layers. The importance of theseconsiderations typically depends on the desired optical properties anduses for the optical bodies. One consideration is the glass transitiontemperature of the second optical layers. Typically, the materials ofthe first and second optical layers are selected so that the glasstransition temperature of the second optical layers is not substantiallyhigher than the glass transition temperature of the first opticallayers. More preferably, the glass transition temperature of the secondoptical layers is equal to or less than the glass transition temperatureof the first optical layers. If the glass transition temperature of thesecond optical layers is too high, orientation of the first opticallayers at a suitable orientation temperature near the glass transitiontemperature of the first optical layers can cause excessive strainhardening in the second optical layers. This can diminish the opticalquality of the second optical layers by, for example, introducing cracksor microvoids. The glass transition temperature of an optical layer isdefined as the glass transition temperature of the composition that isused to form the optical layer and not the glass transitiontemperature(s) of the individual components.

Another consideration is the difference in the z-axis refractive indicesbetween the first and second optical layers. When the z-axis refractiveindices of the two layers are equal, the reflectance of p-polarizedlight does not depend on the incident angle of light. This feature canbe useful when reflectance uniformity over a range of viewing angles isdesired. In such embodiments, the difference in z-axis refractiveindices between the first and second optical layers is preferably nomore than about 0.04 and, by selection of materials can be made no morethan about 0.02 or no more than about 0.01.

Another consideration is the decomposition temperature of the polymer(s)selected for use in the second optical layers. Typical coextrusionprocessing temperatures for PET are above about 250° C. Degradation ofthe components of the second optical layers can produce defects in theoptical body, such as, for example, discoloration and regions of gelformation. Materials that do decompose at the processing temperaturescan still be used if the decomposition does not result in unacceptableproperties.

The second optical layers can be made using a variety of polymericcompositions. The description of suitable polymers with respect toparticular optical body configurations is provided below.

One aspect of this invention utilizespolymethylmethacrylate/polyvinylidene fluoride blends (PMMA/PVDF) in thesecond optical layers as low refractive index materials. The PMMA/PVDFblends are particularly useful with polyester high refractive indexmaterials, for example aromatic polyesters such as polyethyleneterephthalate (PET), as well as blends and copolymers thereof.

The polymers used in multilayer optical films should be clear so thatlight is not lost by scattering or absorption. Many applications involvemultiple interactions between light and the optical film, which magnifythe adverse affects of scattering and absorption. Optical polymers suchas PMMA are considered sufficiently clear for most purposes, withtransmission in the visible region of the spectrum at 92%. PVDF has atransmission of 96%. PMMA/PVDF miscible blends have higher transmission(clarity) than PMMA.

PMMA/PVDF miscible blends have a lower refractive index than PMMA(n=1.49) due to the low index of PVDF (n=1.42). The larger indexdifference results in greater optical power in the multilayer film. Therefractive index for a PMMA/PVDF(60/40) (by weight) miscible blend isabout 1.458. The larger index difference provided by the PMMA/PVDF blendrelative to PMMA also results in a significant dampening of color leaksas well as higher reflectivity.

Multilayer films using PET require high coextrusion temperatures(greater than or equal to about 250° C.) due to the high melting pointsof these polyesters. Second optical layers that are not thermally stablecan cause flow instabilities in the multilayer film due to viscosityloss associated with degradation. Degradation products also may resultin point defects or discoloration in the optical film. PMMA/PVDFmiscible blends are more thermally stable than PMMA.

Thus, by blending PVDF with PMMA, a low refractive index material (forthe second optical layers) with improved properties is achieved. Suchblends have a lower refractive index and a lower glass transitiontemperature as compared to PMMA, while at least maintaining suitableperformance in properties such as clarity, viscosity, thermal stabilityand interlayer adhesion. In particular, the blends, when coextruded withPET as the high refractive index material (for the first opticallayers), exhibit properties such as excellent clarity (e.g.transmission >90%), low refractive index (n≦1.49), viscosity similar tothat of the high refractive index material, thermal stability attemperatures greater than 250° C., glass transition temperature (T_(g))below that of the high refractive index material; and good interlayeradhesion with the high refractive index material.

The particular PMMA and PVDF used in the blends to provide a lowrefractive index material are not limited so long as the materials aresufficiently miscible with each other and the resultant blend can becoextruded with the high refractive index material to form themultilayer film. For example, PMMA sold under the designations Perspex™CP80 and CP82 by ICI Americas, Inc. (Wilmington, Del.) and PVDF soldunder the designation Solef™ 1008/0001 by Solway are useful with PEThigh refractive index materials.

The amount of PVDF used in the blends is more than and not equal toabout 40% by weight and more typically about 50% by weight (i.e. a 50/50PMMA/PVDF blend).

Non-Optical Layers

One or more of the non-optical layers can be formed as a skin layer orskin layers over at least one surface of a stack to, for example,protect the optical layers from physical damage during processing and/orafterwards. In addition or alternatively, one or more of the non-opticallayers can be formed within the stack of layers to, for example, providegreater mechanical strength to the stack or to protect the stack duringprocessing.

The non-optical layers ideally do not significantly participate in thedetermination of optical properties of the multilayer optical film, atleast across the wavelength region of interest (e.g., visible, IR or UVwavelength regions). The non-optical layers may or may not bebirefringent or orientable. Typically, when the non-optical layers areused as skin layers there will be at least some surface reflection. Inat least some applications where high transmission of light is desired,the non-optical layers preferably have an index of refraction that isrelatively low (e.g., no more than 1.6 or, preferably, no more than 1.5)to decrease the amount of surface reflection (e.g., iridescence). Inother applications where reflectivity of light is desired, thenon-optical layers preferably have a relatively high refractive index(e.g., at least 1.6, more preferably at least 1.7) to increasereflectance of the multilayer optical film.

When the non-optical layers are found within the stack, there willtypically be at least some polarization or reflection of light by thenon-optical layers in combination with the optical layers adjacent tothe non-optical layers. In at least some instances, however, thenon-optical layers can be selected to have a thickness that dictatesthat light reflected by the non-optical layers within the stack has awavelength outside the region of interest, for example, in the infraredregion for optical bodies that reflect visible light. The thickness ofthe non-optical layers can be at least two times, typically at leastfour times, and, in many instances, at least ten times, the thickness ofone of the individual optical layers. The thickness of the non-opticallayers can be varied to make an optical film having a particularthickness. Typically, one or more of the non-optical layers are placedso that at least a portion of the light to be transmitted, polarized,and/or reflected by the optical layers, also travels through thenon-optical layers (i.e., the non-optical layers are placed in the pathof light which travels through or is reflected by the optical layers).

The non-optical layers are formed from polymers including any of thepolymer used in the first and second optical layers. In someembodiments, the material selected for the non-optical layers is similarto or the same as the material selected for the second optical layers.Materials may be chosen for the non-optical layers that impart orimprove properties such as, for example, tear resistance, punctureresistance, toughness, weatherability, and solvent resistance of themultilayer optical film.

Other Layers and Coatings

Various functional layers or coatings can be added to the multilayeroptical films of the present invention to alter or improve theirphysical or chemical properties, particularly along the surface of themultilayer optical film. Such layers or coatings may include, forexample, slip agents, low adhesion backside materials, conductivelayers, antistatic coatings or films, barrier layers, flame retardants,UV stabilizers, abrasion resistant materials, optical coatings, and/orsubstrates designed to improve the mechanical integrity or strength ofthe film or device, as described in WO 97/01440, which is hereinincorporated by reference. Dichroic polarizing films can also be coatedon or co-extruded with the multilayer optical films, as described, forexample, in WO 95/17691, WO 99/36813, and WO 99/36814, all of which areherein incorporated by reference.

Manufacturing

A brief description of one method for forming multilayer optical filmsis provided. A fuller description of the process conditions andconsiderations is found in PCT Publications Nos. WO 99/36248, WO99/06203, and WO 99/36812, all of which are incorporated herein byreference.

An initial step in the manufacture of the multilayer optical films isthe generation of the polymers to be used in formation of the first andsecond optical layers, as well as the non-optical layers (unless thepolymers are available commercially). Typically, these polymers areformed by extrusion, although other methods of polymer formation can maybe used. Extrusion conditions are chosen to adequately feed, melt, mixand pump the polymer resin feed streams in a continuous and stablemanner. Final melt stream temperatures are chosen to be within a rangethat reduces freezing, crystallization, or unduly high pressure drops atthe low end of the range and that reduces degradation at the high end ofthe range. The entire melt stream processing of more than one polymer,up to and including film casting on a chill roll, is often referred toas co-extrusion.

Preferably, the polymers of the first optical layers, the second opticallayers, and the non-optical layers are chosen to have similarrheological properties (e.g., melt viscosities) so that they can beco-extruded. Typically, the second optical layers and the non-opticallayers have a glass transition temperature, T_(g), that is either belowor no greater than about 30° C. above the glass transition temperatureof the first optical layers. Preferably, the glass transitiontemperature of the second optical layers and the non-optical layers isbelow the glass transition temperature of the first optical layers.

Following extrusion, each melt stream is conveyed to a gear pump used toregulate the continuous and uniform rate of polymer flow. A staticmixing unit can be used to carry the polymer melt stream from the gearpump into a multilayer feedblock with uniform melt stream temperature.The entire melt stream is typically heated as uniformly as possible toenhance both uniform flow of the melt stream and reduce degradationduring melt processing.

Multilayer feedblocks divide each of the two or more polymer meltstreams into many layers, interleave these layers, and combine the manylayers into a single multilayer stream. The layers from any given meltstream are created by sequentially bleeding off part of the stream froma main flow channel into side channel tubes which lead to layer slots inthe feed block manifold. The layer flow can be controlled by choicesmade in machinery, as well as the shape and physical dimensions of theindividual side channel tubes and layer slots.

The side channel tubes and layer slots of the two or more melt streamsare often interleaved to form alternating layers. The feedblock'sdownstream-side manifold is typically shaped to compress and uniformlyspread the layers of the combined multilayer stack transversely. Thick,non-optical layers, known as protective boundary layers (PBLs), can befed near the manifold walls using the melt streams of the opticalmultilayer stack, or by a separate melt stream. As described above,these non-optical layers can be used to protect the thinner opticallayers from the effects of wall stress and possible resulting flowinstabilities.

The multilayer stack exiting the feedblock manifold enters a finalshaping unit such as a die. Alternatively, the stream can be split,preferably normal to the layers in the stack, to form two or moremultilayer streams that can be recombined by stacking. The stream canalso be split at an angle other than normal to the layers. A flowchanneling system that splits and stacks the streams is called amultiplier. The width of the split streams (i.e., the sum of thethicknesses of the individual layers) can be equal or unequal. Themultiplier ratio is defined as the ratio of the wider to narrower streamwidths. Unequal streams widths (i.e., multiplier ratios greater thanunity) can be useful in creating layer thickness gradients. In the caseof unequal stream widths, the multiplier may spread the narrower streamand/or compress the wider stream transversely to the thickness and flowdirections to ensure matching layer widths upon stacking.

Prior to multiplication, additional non-optical layers can be added tothe multilayer stack. These non-optical layers may perform as PBLswithin the multiplier. After multiplication and stacking, some of theselayers can form internal boundary layers between optical layers, whileothers form skin layers.

After multiplication, the web is directed to a final shaping unit. Theweb is then cast onto a chill roll, sometimes also referred to as acasting wheel or casting drum. This casting is often assisted byelectrostatic pinning, the details of which are well-known in the art ofpolymer film manufacture. The web can be cast to a uniform thicknessacross the web or a deliberate profiling of the web thickness can beinduced using die lip controls.

The multilayer web is then uniaxially or biaxially drawn to produce thefinal multilayer optical film. Uniaxial drawing is performed in a tenteror a length orienter. Biaxial drawing typically includes both types ofequipment. Typical tenters draw in a transverse direction (TD) to theweb path, although certain tenters are equipped with mechanisms to drawor relax (shrink) the film dimensionally in the web path or machinedirection (MD). Length orienters draw in the machine direction.

For example, a two step drawing process is used to orient thebirefringent material in both in-plane directions. The draw processescan be any combination of the single step processes described above thatallow drawing in two in-plane directions. In addition, a tenter thatallows drawing along the machine direction, e.g. a biaxial tenter whichcan draw in two directions sequentially or simultaneously, can be used.In this latter case, a single biaxial draw process can be used.

In some embodiments, the film is annealed prior to subsequent processingor use. Typically the film is annealed at an annealing temperature ofbetween 70 and 95 degrees C. Typically the film is annealed for at least20 seconds at a temperature of between 80 and 95 degrees C. Moretypically the film is annealed for at least 25 seconds at a temperatureof between 80 and 95 degrees C. More typically the film is annealed forat least 30 seconds at a temperature of between 80 and 95 degrees C.More typically the film is annealed for at least 35 seconds at atemperature of between 80 and 95 degrees C. In some embodiments the filmis annealed for at least two minutes at a temperature of between 70 and95 degrees centigrade. In some embodiments the film is annealed for atleast one hour at a temperature of between 70 and 95 degrees centigrade.In some embodiments the film is held under no tension in any dimensionduring annealing. In some embodiments the film is held under no tensionin at least one in-plane dimension during annealing. In some embodimentsthe film is held under no tension in one in-plane dimension and lowtension in a perpendicular in-plane dimension during annealing.

In some embodiments, the multilayer polymeric reflector according to thepresent disclosure is used to form a reflective panel for mounting on anarchitectural structure such as a building, walkway, bridge, or thelike. The panel may comprise a multilayer polymeric reflector accordingto the present disclosure and support elements adapted for mounting toan architectural structure. Support elements may include elementsselected from framing members, mounting brackets, adhesive, connectors,mounting pins, anchors, and the like, and any combination thereof. Themultilayer polymeric reflector may be semi-specular at visiblewavelengths. In some embodiments, the multilayer polymeric reflector isat least 50% specular at visible wavelengths. In some embodiments, themultilayer polymeric reflector is no more than 90% specular at visiblewavelengths. In some embodiments, the multilayer polymeric reflector isno more than 80% specular at visible wavelengths. In some embodiments,the multilayer polymeric reflector is no more than 70% specular atvisible wavelengths. In some embodiments, two or more reflective panelsare mounted on opposing sides of a gap formed between two architecturalstructures or two portions of a single architectural structure so as toincrease the propagation of sunlight from the top of the gap tolocations farther down the gap. In some embodiments the gap is acourtyard, airshaft, or recess in a single building. In some embodimentsthe gap is a courtyard, airshaft, recess, alleyway or separation formedbetween two or more architectural structures. In some embodiments, twoor more reflective panels are mounted on at least two opposing surfaces.In some embodiments, reflective panels are mounted on at least 50% ofthe area of at least two opposing surfaces. In some embodiments, two ormore reflective panels are mounted on at least two opposing andsubstantially parallel surfaces. In some embodiments, reflective panelsare mounted on at least 50% of the area of at least two opposing andsubstantially parallel surfaces. In some embodiments, four or morereflective panels are mounted on at least two pair of opposing surfaces.In some embodiments, reflective panels are mounted on at least 50% ofthe area of at least two pair of opposing surfaces. In some embodiments,four or more reflective panels are mounted on at least two pair ofopposing and substantially parallel surfaces. In some embodiments,reflective panels are mounted on at least 50% of the area of at leasttwo pair of opposing and substantially parallel surfaces.

Selected Embodiments

The following numbered embodiments are intended to further illustratethe present disclosure but should not be construed to unduly limit thisdisclosure.

-   1. A multilayer polymeric reflector comprising:    -   a) a plurality of first optical layers, each first optical layer        comprising a polyester having terephthalate comonomer units and        ethylene glycol comonomer units, said polyester having a glass        transition temperature, wherein each first optical layer is        oriented, and    -   b) a plurality of second optical layers disposed in a repeating        sequence with the plurality of first optical layers, each second        optical layer comprising a blend of polymethyl methacrylate        (PMMA) and polyvinylidene fluoride (PVDF), wherein said blend        has a glass transition temperature less than the glass        transition temperature of the polyester comprising the first        optical layers, and wherein the amount of PVDF in the PMMA/PVDF        blend is greater than and not equal to about 40% and not more        than about 65%;        wherein the multilayer polymeric reflector has a reflectivity of        greater than 97.8% in a visible wavelength region and a        transmission haze value of less than 50% in a visible wavelength        region.-   2. The multilayer polymeric reflector according to embodiment 1    wherein the amount of PVDF in the PMMA/PVDF blend is greater than    45%.-   3. The multilayer polymeric reflector according to embodiment 1    wherein the amount of PVDF in the PMMA/PVDF blend is greater than or    equal to about 50%.-   4. The multilayer polymeric reflector according to embodiment 1    wherein the amount of PVDF in the PMMA/PVDF blend is about 50%.-   5. The multilayer polymeric reflector according to any of    embodiments 1-4, wherein the multilayer polymeric reflector has a    reflectivity of greater than 98.0%.-   6. The multilayer polymeric reflector according to any of    embodiments 1-4, wherein the multilayer polymeric reflector has a    reflectivity of greater than 98.2%.-   7. The multilayer polymeric reflector according to any of    embodiments 1-6 wherein the total number of first and second layers    is no more than 700.-   8. The multilayer polymeric reflector according to any of    embodiments 1-6 wherein the total number of first and second layers    is no more than 650.-   9. The multilayer polymeric reflector according to any of    embodiments 1-8, wherein the multilayer polymeric reflector resists    shrinkage in use, to the extent that it demonstrates shrinkage of    less than 1.5% in the total of width plus length following an    exposure of 15 minutes to a temperature of 120 degrees centigrade.-   10. The multilayer polymeric reflector according to embodiment 9,    wherein the multilayer polymeric reflector demonstrates shrinkage of    less than 1.0%.-   11. The multilayer polymeric reflector according to embodiment 9,    wherein the multilayer polymeric reflector demonstrates shrinkage of    less than 0.5%.-   12. The multilayer polymeric reflector according to embodiment 9,    wherein the multilayer polymeric reflector demonstrates shrinkage of    less than 0.2%.-   13. The multilayer polymeric reflector according to any of    embodiments 1-12, wherein the multilayer polymeric reflector has a    transmission haze value of less than 46% in a visible wavelength    region.-   14. The multilayer polymeric reflector according to embodiment 13,    wherein the multilayer polymeric reflector has a transmission haze    value of less than 42% in a visible wavelength region.-   15. The multilayer polymeric reflector according to embodiment 13,    wherein the multilayer polymeric reflector has a transmission haze    value of less than 30% in a visible wavelength region.-   16. The multilayer polymeric reflector according to embodiment 13,    wherein the multilayer polymeric reflector has a transmission haze    value of less than 20% in a visible wavelength region.-   17. The multilayer polymeric reflector according to embodiment 13,    wherein the multilayer polymeric reflector has a transmission haze    value of less than 10% in a visible wavelength region.-   18. The multilayer polymeric reflector according to any of    embodiments 1-5, wherein the first and second optical layers are    coextruded.-   19. The multilayer polymeric reflector according to any of    embodiments 1-18, wherein the first optical layers are biaxially    oriented.-   20. The multilayer polymeric reflector according to any of    embodiments 1-19, wherein the multilayer polymeric reflector is    annealed at an annealing temperature of between 70 and 95 degrees    centigrade for at least 30 seconds.-   21. The multilayer polymeric reflector according to embodiment 20,    wherein the multilayer polymeric reflector is annealed at an    annealing temperature of between 80 and 95 degrees centigrade for at    least 30 seconds.-   22. The multilayer polymeric reflector according to embodiment 20,    wherein the multilayer polymeric reflector is annealed at an    annealing temperature of between 80 and 95 degrees centigrade for at    least 35 seconds.-   23. The multilayer polymeric reflector according to embodiment 20,    wherein the multilayer polymeric reflector is annealed at an    annealing temperature of between 70 and 95 degrees centigrade for at    least two minutes.-   24. The multilayer polymeric reflector according to embodiment 20,    wherein the multilayer polymeric reflector is annealed at an    annealing temperature of between 70 and 95 degrees centigrade for at    least one hour.-   25. The multilayer polymeric reflector according to any of    embodiments 1-24, additionally comprising an optically clear    UV-rejecting acrylic coating layer.-   26. The multilayer polymeric reflector according to any of    embodiments 1-25, additionally comprising an adhesive layer.-   27. The multilayer polymeric reflector according to any of    embodiments 1-26, which is specular or semi-specular at visible    wavelengths.-   28. The multilayer polymeric reflector according to any of    embodiments 1-26, which is at least 50% specular at visible    wavelengths.-   29. An article comprising:    -   a) a light source; and    -   b) the multilayer polymeric reflector according to any of        embodiments 1-28 situated so as to reflect light emitted by the        light source.-   30. The article according to embodiment 29 wherein the light source    is an LED.-   31. The article according to embodiment 29 or 30 which is a    luminaire.-   32. The article according to embodiment 29 or 30 which is a light    bulb.-   33. An article comprising the multilayer polymeric reflector    according to any of embodiments 1-28 situated so as to receive and    reflect sunlight.-   34. An article comprising the multilayer polymeric reflector    according to any of embodiments 1-28 situated so as to receive and    reflect direct sunlight.-   35. An article comprising the multilayer polymeric reflector    according to any of embodiments 1-28 situated so as to receive and    reflect concentrated sunlight.-   36. The article according to any of embodiments 29-35 which is    designed for outdoor use.-   37. The article according to any of embodiments 29-35 installed for    outdoor use.-   38. The article according to any of embodiments 29-37 wherein the    multilayer polymeric reflector is directly exposed to ambient    outdoor light.-   39. The article according to any of embodiments 29-37 wherein the    multilayer polymeric reflector is directly exposed to ambient    outdoor light and ambient outdoor air.-   40. The film according to any of embodiments 1-28 installed for    outdoor use.-   41. The film according to embodiment 40 wherein the multilayer    polymeric reflector is directly exposed to ambient outdoor light.-   42. The film according to embodiment 40 wherein the multilayer    polymeric reflector is directly exposed to ambient outdoor light and    ambient outdoor air.-   43. A reflective panel which is the article according to claim 33    additionally comprising support elements adapted for mounting to an    architectural structure.-   44. The reflective panel according to claim 43 wherein the    multilayer polymeric reflector is at least 50% specular at visible    wavelengths and no more than 90% specular at visible wavelengths.-   45. The reflective panel according to claim 43 wherein the    multilayer polymeric reflector is at least 50% specular at visible    wavelengths and no more than 80% specular at visible wavelengths.-   46. An arrangement of two or more reflective panels articles    according to any of claims 43-45, wherein two or more reflective    panels are mounted on at least two opposing surfaces of an    architectural structure.-   47. An arrangement of two or more reflective panels articles    according to any of claims 43-45, wherein two or more reflective    panels are mounted on at least two opposing surfaces of two or more    architectural structures.-   48. An arrangement of two or more reflective panels articles    according to any of claims 43-45, wherein reflective panels are    mounted on at least 50% of at least two opposing and substantially    parallel surfaces an architectural structure.-   49. An arrangement of two or more reflective panels articles    according to any of claims 43-45, wherein reflective panels are    mounted on at least 50% of at least two opposing and substantially    parallel surfaces two or more architectural structures.-   50. An arrangement of two or more reflective panels articles    according to any of claims 43-45, wherein reflective panels are    mounted on at least 50% of at least two pair of opposing and    substantially parallel surfaces.

Objects and advantages of this disclosure are further illustrated by thefollowing examples, but the particular materials and amounts thereofrecited in these examples, as well as other conditions and details,should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all reagents were obtained or are available fromAldrich Chemical Co., Milwaukee, Wis., or may be synthesized by knownmethods.

The following films were made by coextrusion and biaxial orientation oftwo optical layer materials according to methods described above.

“ESR” is a comparative film composed of 265 PEN first optical layersalternating in an interleaved fashion with 265 PMMA second opticallayers, additionally composed of PEN outer skin layers, having areflectivity of 99.50% and a physical caliper of about 2.6 mil.

“DESR” is a comparative film composed ESR and an additional acrylic UVcoating.

“PETb-ESR” is an exemplary film composed of 325 PET first optical layersalternating in an interleaved fashion with 325 PMMA/PVDF (50/50) secondoptical layers, additionally composed of PET outer skin layers, having areflectivity of about 98% and a physical caliper of about 3.2 mil.

“PETb-DESR” is an exemplary film composed PETb-ESR and an additionalacrylic UV coating.

Average Reflectance

Reflectance was measured in the visible range according to ASTME1164-02/E308-01 using a Lambda 1050 Spectrometer for 4 differentexamples of PETb-ESR and for two comparative films: “PETb-ESR30”, whichwas similar to PETb-ESR except that the second optical layers werePMMA/PVDF (70/30), and “PETb-ESR40”, which was similar to PETb-ESRexcept that the second optical layers were PMMA/PVDF (60/40). None ofthe films were annealed or heat treated. Results are reported in TableI.

TABLE I Example Average Reflectance PETb-ESR (Average of four) ~98.2%PETb-ESR40 (Comparative) ~97.2% PETb-ESR30 (Comparative) ~96.2%

UV Light Resistance of PET Based ESR Films

UV light resistance was measured under black light UV exposure (radiantflux 87 W/m² with 340 nm peak) for comparative ESR and DESR films andexemplary PETb-ESR (two examples) and PETb-DESR films. UV exposure wason the reflective side of the reflector. Yellowing (change in b*) wasmeasured on the reflective side of the reflector using aspectrophotometer (model CM-5) made by Konica Minolta. The results arereported in the graph of FIG. 1.

The comparative PEN based film “ESR” yellowed noticeably (b* increasedby 4) in 25 hours. For the exemplary PET based film “PETb-ESR”, b*increased by only 3.2 after exposure for more than 56 days. For theexemplary PET based film with UV coating, “PETb-DESR”, b* increasednegligibly even after 56 days under black light.

High Intensity Visible Light Durability

The comparative PEN based film “ESR” and exemplary PET based film“PETb-ESR” were additionally tested for durability under high intensityLED-source white light and blue light. The exemplary PET based film“PETb-ESR” withstood 80 times longer exposure without noticeableyellowing in comparison to the comparative PEN based film “ESR”.

Reduced Shrinkage by Heat Stabilization Process

Four films were tested for shrinkage, comparative film ESR (designated“Regular ESR” in FIG. 2), and three exemplary films: PETb-ESR(designated “PETb-ESR (untreated)” in FIG. 2), and two heat stabilizedsamples of PETb-ESR, designated “PETb-ESR (HS1)” and “PETb-ESR (HS2)”.PETb-ESR (HS1) was pretreated by heating to 140 degrees C. for 5minutes. PETb-ESR (HS2) was pretreated by heating to 150 degrees C. for30 minutes. All four were measured for shrinkage after a 15 minuteexposure to 85 degrees C., 100 degrees C., and 120 degrees C.temperatures. The results are reported in the graph of FIG. 2.

Without heat stabilization, the PET based PETb-ESR film has adisadvantage relative to the comparative PEN based ESR in that itdemonstrates higher shrinkage at temperature above 60° C. This isthought to be due to low glass transition temperature of PET (70 C) andlow glass transition temperature of PMMA/PVDF blend (50 C). The presentdisclosure contemplates annealing PET based films by heating up to 20˜30degree C. higher than the target application temperature at low tensionor no tension.

Haze Measurements

Haze values were measured for various films by Hazegard+. Haze valueswere measured for light transmitted through the film. Unless otherwisespecified, haze was measured at the back side of the film (“back sidemeasurement”), with a light source at the reflective side of the film.Alternately, haze can be measured at the reflective side of the film(“reflective side measurement”), with a light source at the back side ofthe film. The graph of FIG. 3 reports results for an untreated PETb-ESRfilm (designated “off A3 line”) and PETb-ESR films subjected to variousheat histories. It can readily be seen that the samples annealed at 85degrees C. resisted hazing.

Additional haze values were measured for PETb-ESR40, which are reportedin Table II.

TABLE II Reflective Side Back Side Measurement Measurement Beforeheating 5.8 12.5 Heated to 150 C. 10.1 19.4 for 5 min

Additional haze values were measured for PETb-ESR after processingthrough a three-stage oven. The three stage temperatures were set to 82degrees C., 148 degrees C. and 148 degrees C., to provide annealing andsubsequent heat stabilization. Measured haze values are reported inTable III.

TABLE III Reflective Back 82 degree C. 148 degree C. Side Haze Side Hazedwell time dwell time Measurement Measurement 180 sec 450 sec 23.5 43.690 sec 225 sec 29.6 49.2 51 sec 129 sec 25.6 45.5 36 sec 90 sec 23.741.2

Clear UV Coat Layer Found to Eliminate OligomerMigration/Crystallization

PETb-ESR and PETb-DESR samples were held at 150 degrees C. for 15 hoursand examined for PET surface haze. It is believed that the mechanism offormation for this type of haze is the migration of PET oligomercrystals to the surface of the outermost PET layer of the film. Asdemonstrated in FIG. 4, application of the clear UV coat eliminated thismechanism of hazing.

Outdoor Exposure Testing

Comparative ESR and DESR films and exemplary PETb-ESR and PETb-DESRfilms were subjected to outdoor exposure testing at New River, Ariz.,USA, using test methods ASTM G147-2009 and ASTM G90-2010 (no waterspray) over an 83 day period. Radiant Energy measured was:

-   -   UV: 270 MJ/m2 (295˜385 nm)    -   Total radiation: 10,592 MJ/m2

b* and photopic weighted reflectance were measured before and afterexposure. The results are reported in Table IV.

TABLE IV Photopic weighted b* (yellowing) reflectance (%) Exampleinitial final change initial final change ESR 0.5 15.7 +15.3 99.2 91.5−7.7 (comparative) DESR 0.7 4.9 +4.2 99.2 98   −1.2 (comparative)PETb-ESR −0.1 1.4 +1.5 98.9 98.2 −0.7 (exemplary) PETb-DESR 0.1 0.5 +0.498.9 98.7 −0.2 (exemplary)

Various modifications and alterations of this disclosure will becomeapparent to those skilled in the art without departing from the scopeand principles of this disclosure, and it should be understood that thisdisclosure is not to be unduly limited to the illustrative embodimentsset forth hereinabove.

We claim:
 1. A multilayer polymeric optical film comprising a pluralityof alternating first and second polymeric optical layers, each first andsecond optical layer reflecting and transmitting light by coherentinterference of light, the optical film having a reflectivity of greaterthan about 97.8% and a transmission haze value of less than about 50% ina visible wavelength region, such that when exposed to UV light having aradiant flux of about 87 W/m² with a peak radiation of about 340 nm forabout 56 days, a yellow index b* of the optical film increases by lessthan 4, and wherein when exposed to a temperature of about 120 degreescentigrade for about 15 minutes, the optical film shrinks less than1.5%.
 2. The multilayer polymeric optical film of claim 1 furthercomprising a non-optical layer disposed within the plurality ofalternating first and second optical layers.
 3. The multilayer polymericoptical film of claim 1 further comprising a non-optical layer disposedon the plurality of alternating first and second optical layers.
 4. Themultilayer polymeric optical film of claim 1, wherein at least one firstoptical layer comprises in-plane refractive indices n_(1x) and n_(1y)and at least one second optical layer comprises in-plane refractiveindices n_(2x) and n_(2y), and wherein n_(1x)≠n_(2x) and n_(1y)≠n_(2y).5. The multilayer polymeric optical film of claim 1, wherein at leastone first optical layer comprises in-plane refractive indices n_(1x) andn_(1y) and at least one second optical layer comprises in-planerefractive indices n_(2x) and n_(2y), and wherein n_(1x)≠n_(2x) andn_(1y) is approximately equal to n_(2y).
 6. The multilayer polymericoptical film of claim 5, wherein a difference between n_(1y) and n_(2y)is less than about 0.04.
 7. The multilayer polymeric optical film ofclaim 1, wherein at least one first optical layer is birefringent. 8.The multilayer polymeric optical film of claim 1, wherein at least onefirst optical layer is uniaxially oriented.
 9. The multilayer polymericoptical film of claim 1, wherein at least one first optical layer isbiaxially oriented.
 10. The multilayer polymeric optical film of claim1, wherein at least one second optical layer is birefringent.
 11. Themultilayer polymeric optical film of claim 1, wherein at least onesecond optical layer is isotropic.
 12. The multilayer polymeric opticalfilm of claim 1, wherein each first optical layer comprises a polyesterhaving terephthalate comonomer units and ethylene glycol comonomerunits.
 13. The multilayer polymeric optical film of claim 1, whereineach second optical layer comprises a blend of polymethyl methacrylate(PMMA) and polyvinylidene fluoride (PVDF).
 14. The multilayer polymericoptical film of claim 13, wherein an amount of PVDF in the PMMA/PVDFblend is greater than about 40% and not more than about 65%.
 15. Themultilayer polymeric optical film of claim 13, wherein a glasstransition of each second optical layer is less than a glass transitionof each first optical layer.
 16. An article comprising the multilayerpolymeric optical film of claim 1 situated so as to receive and reflectsunlight.
 17. An article comprising the multilayer polymeric opticalfilm of claim 1 situated so as to receive and reflect direct sunlight.18. An article comprising the multilayer polymeric optical film of claim1 situated so as to receive and reflect concentrated sunlight.